Coronavirus
lec -15
Diseases
Coronaviruses are an important cause of the common cold. In 2002, a new
disease, an atypical pneumonia called SARS (severe acute respiratory syndrome)
emerged.
Important Properties
Coronaviruses have a nonsegmented, single-stranded, positive-polarity RNA
genome. They are enveloped viruses with a helical nucleocapsid. There is no
virion polymerase. In the electron microscope, prominent club-shaped spikes in
the form of a "corona" (halo) can be seen.
Virion: Spherical&helical nucleocapsid
Genome: Single-stranded RNA, linear, nonsegmented&positive-sense.
Proteins: Two glycoproteins and one phosphoprotein.
Envelope: Contains large, widely spaced, club- or petal-shaped spikes
Replication: Cytoplasm; particles mature by budding into endoplasmic reticulum
and Golgi
Outstanding characteristics:
Cause colds and SARS
Difficult to grow in cell culture
Transmission & Epidemiology
Coronaviruses are transmitted by the respiratory aerosol. SARS originated in
China in November 2002 and spread rapidly to other countries. As of this
writing, there have been 8300 cases and 785 deaths. Human-to-human
transmission occurs, and some patients with SARS are thought to be "superspreaders," .
Pathogenesis & Clinical Findings
-Coronavirus infection is typically limited to the mucosal cells of the respiratory
tract. infections are asymptomatic, and it is unclear what role they play in the
spread of infection.
-The common cold caused by coronavirus is characterized by coryza (rhinorrhea,
runny nose), sore throat, and low-grade fever. This illness typically lasts several
days. Coronaviruses also cause bronchitis.
SARS is a severe atypical pneumonia characterized by a fever of at least 38°C,
nonproductive cough, dyspnea, and hypoxia. Chills, rigors, malaise, and
headache commonly occur. Leukopenia and thrombocytopenia are seen.
Picornaviruses--ENTEROVIRUSES --Poliovirus
Disease
This virus causes poliomyelitis.
Important Properties
The host range is limited to primates, i.e., humans and nonhuman primates such
as monkeys. This limitation is due to the binding of the viral capsid protein to a
receptor found only on primate cell membranes.
There are three serologic (antigenic) types based on different antigenic .
Virion: Icosahedral& 28–30 nm in diameter
Genome: Single-stranded RNA, linear& positive-sense
Envelope: None
Replication: Cytoplasm
Transmission
Poliovirus is transmitted by the fecal–oral route. It replicates in the oropharynx
and intestinal tract. Humans are the only natural hosts.
As a result of the success of the vaccine, poliomyelitis caused by naturally
occurring "wild-type" virus has been eradicated.
Pathogenesis
After replicating in the oropharynx and small intestine, especially in lymphoid
tissue, the virus spreads through the bloodstream to the central nervous system. It
can also spread retrograde along nerve axons.
In the central nervous system, poliovirus replicates in the motor neurons located
in the anterior horn of the spinal cord. Death of these cells results in paralysis of
the muscles innervated by those neurons. Paralysis is not due to virus infection of
muscle cells. The virus also affects the brain stem, leading to "bulbar"
poliomyelitis (with respiratory paralysis), but rarely damages the cerebral cortex.
Clinical Findings
The range of responses to poliovirus infection includes
-(1) inapparent, asymptomatic infection;Asymptomatic infection is quite
common. The incubation period is usually 10–14 days.
-(2) abortive poliomyelitis;The most common clinical form is abortive
poliomyelitis, which is a mild, febrile illness characterized by headache, sore
throat, nausea, and vomiting. Most patients recover spontaneously.
-(3) nonparalytic poliomyelitis; Nonparalytic poliomyelitis manifests as aseptic
meningitis with fever, headache, and a stiff neck. This also usually resolves
spontaneously.
-(4) paralytic poliomyelitis: In paralytic poliomyelitis, flaccid paralysis is the
predominant finding, but brain stem involvement can lead to life-threatening
respiratory paralysis. Painful muscle spasms also occur. The motor nerve damage
is permanent, but some recovery of muscle function occurs as other nerve cells
take over. In paralytic polio, both the meninges and the brain parenchyma
(meningoencephalitis) are often involved. If the spinal cord is also involved, the
term meningomyeloencephalitis is often used.
Prevention
Poliomyelitis can be prevented by both the killed vaccine (Salk vaccine,
inactivated vaccine, IPV) and the live, attenuated vaccine (Sabin vaccine, oral
vaccine, OPV) Both vaccines induce humoral antibodies, which neutralize virus
entering the blood and hence prevent central nervous system infection and
disease. Both the killed and the live vaccines contain all three serotypes.
Norwalk Virus (Norovirus)
Disease
Norwalk virus (also known as Norovirus) is one of the most common causes of
viral gastroenteritis in adults worldwide. It is named for an outbreak of
gastroenteritis in a school in Norwalk, Ohio, in 1969.
Important Properties
Norwalk virus has a nonsegmented, single-stranded, positive-polarity RNA
genome. It is a nonenveloped virus with an icosahedral nucleocapsid. There is no
virion polymerase. In the electron microscope, 10 prominent spikes and 32 cupshaped depressions can be seen. The number of serotypes is uncertain.
Summary of Replicative Cycle
Norwalk virus has not been grown efficiently in cell culture, so its replicative
cycle has been difficult to study. It is presumed to replicate in a manner similar to
that of picornaviruses.
Transmission
Norwalk virus is transmitted by the fecal–oral route, often involving the ingestion
of contaminated seafood or water. Outbreaks typically occur in group settings
such as schools, camps, hospitals, and nursing homes. Person-to-person
transmission also occurs, especially in group settings.
Pathogenesis & Clinical Findings
-Norwalk virus infection is typically limited to the mucosal cells of the intestinal
tract. Watery diarrhea without red cells or white cells occurs.
-Disease is characterized by sudden onset of vomiting and diarrhea accompanied
by low-grade fever and abdominal cramping. Neither the emesis nor the stool
contain blood. In some outbreaks, certain patients manifest signs of central
nervous system involvement such as headache, meningismus and photophobia.
Family -Reoviruses: ROTAVIRUS
Rotaviruses are the most important human pathogens in the reovirus family.
Disease
Rotavirus is the most common cause of viral gastroenteritis in young children.
Important Properties
Reoviruses, including rotavirus ,There are at least six serotypes of human
rotavirus.
The virion contains an RNA-dependent RNA polymerase. A virion polymerase
is required because human cells do not have an RNA polymerase that can
synthesize mRNA from a double-stranded RNA template.
Virion: Icosahedral, 60–80 nm in diameter, double capsid shell
Genome: Double-stranded RNA, linear, segmented (10–12 segments)
Envelope: None (transient pseudoenvelope is present during rotavirus particle
morphogenesis)
Replication: Cytoplasm; virions not completely uncoated
Outstanding characteristics:
Genetic reassortment occurs readily
Rotaviruses are the major cause of infantile diarrhea
Transmission & Pathogenesis
-Rotavirus is transmitted by the fecal–oral route.
-Rotavirus replicates in the mucosal cells of the small intestine, resulting in the
excess secretion of fluids and electrolytes into the bowel lumen. The consequent
loss of salt, glucose, and water leads to diarrhea. No inflammation occurs, and
the diarrhea is nonbloody. It is thought that this watery diarrhea is caused
primarily by stimulation of the enteric nervous system.
Clinical Findings
Rotavirus infection is characterized by nausea, vomiting, and watery, nonbloody
diarrhea. Gastroenteritis is most serious in young children, in whom
dehydration and electrolyte imbalance are a major concern. Adults usually have
minor symptoms.